1. Field of the Invention
The present invention relates to a MOSFET switch, and more particularly to a MOSFET switch connecting electronic signals including high fidelity audio, other analog as well as digital signals
2. Background Information
MOSFET devices are preferred switches for coupling electronic signals since they demonstrate no offset voltages that may be found in bipolar and other solid state switches, and MOSFET switch on resistance (Ron) is very low and the off resistance (Roff) is very high. In modern devices Ron may be under one ohm and Roff may be many megohms. Note “coupling” and “couples” as used herein includes direct connections.
In MOSFET switches Ron is a function the gate to source voltage, Vgs, of the device. Since Ron is a function of Vgs, Ron will be reasonably constant if Vgs is constant, other conditions being equal.
When such switch device are used for audio signals, the fidelity of the transferred signal across the switch may be negatively affected if Ron changes with the changing of the input signal voltage level.
One measure of the audio fidelity of a switch may be the total harmonic distortion (THD) that is introduced by the switch. One standard measurement of THD includes measuring signal fidelity near 220 KHz which is the 11th harmonic of the high limit of the standard audio range of 20 Hz to 20 KHz.
Keeping a MOSFET Ron constant despite input signal voltage variations is an object of U.S. Pat. No. 4,093,874, to Pollitt. Pollitt discusses keeping Ron constant by keeping Vgs constant (at a set temperature) regardless of changes of the input signal voltage. Pollitt, however, uses the logic signal voltage value, that turns the switch on/off, to determine the Vgs voltage level. But the logic signal voltage level will change as the power supply voltage (that produces the logic signal voltage) changes with loads. The logic signal value may also change with other loads on the logic signal itself. Such changes in Vgs will change Ron and, also, limit the useful dynamic range of the '874 invention.